The present invention relates generally to the field of non-impact printing and, in particular, the present invention reveals a method for automatically creating custom ink and media-dependent transforms so that a print engine operated under computer control is capable of autonomous operation by determining ink color and maximum percent ink coverage of a media (and therefore ink mixing ratios) and applying that knowledge to produce and xe2x80x98sensexe2x80x99 color values of a plurality of colored ink patches; a custom ink and media dependent transform is thus created using the xe2x80x98sensedxe2x80x99 color values of the patches and the transform is then used during printing operations to accurately expel a wide variety of ink colors from a plurality of print heads, at optimum percent ink coverage for a given media, in a large format full-color ink jet print engine.
In the prior art related to ink jet printing a print head operated under precise electronic control typically opposes a portion of a printing media so that an image may be printed thereon. Typically, to achieve printed images of the highest quality each of a plurality of ink emitting elements that emit droplets of colorant onto the printing media need synchronization in respect of their position and orientation with respect to each other such element (i.e., exact xe2x80x9cregistrationxe2x80x9d). In prior art multi-head digital print engines including drum-based, swath (or carriage-based), and flat-bed digital print engines, it is known that consistency of mounting and operation of such elements increases the level of registration among said elements and thus decreases the likelihood of printing errors and image artifacts. In a traditional drum-based print engine a print media attaches to a rotating drum which then passes under one or more discrete ink emitting print elements (xe2x80x9cnozzlesxe2x80x9d) mounted on a carriage articulated in the axial direction. In a flat bed print engine, the printing media is rigidly coupled to a substantially planar surface and the nozzles are articulated in two dimensions to cover the media. In a reciprocating swath, or carriage-based, print engine the media is incrementally stepped over a platen surface in one direction while the nozzles reciprocate across the media in a direction orthogonal to direction the media advances. In many of these traditional print engines perfect registration has become even more difficult to efficiently achieve as the number of print heads and the number of ink emitting elements increase and service and replacement procedures become more frequent. Accordingly, in practical terms it is known that in some businesses specializing in producing full color digitally printed output, time constraints to complete printing jobs will conflict and oftentimes prevail with time required to complete full calibration and registration routines.
Accuracy of color reproduction in printing full-tone images using a plurality of ink heads emitting drops of colored inks on the printing substrate under computer control depends on numerous characteristics of the hardware/software train employed in the printing process, including those of the inks, substrate, ink heads, as well as on certain environmental characteristics such as humidity and temperature. Some of the factors substantially impacting color reproduction may vary significantly and may be volatile and hard to stabilize or characterize to the necessary degree. Because of this, to achieve good accuracy in color reproduction using computer controlled printing devices, it is necessary to measure chromatic characteristics of a large set of color patches printed with a given printing equipment, and to use these measurements as feedback to tune up the printing process. To diminish the number of patches that need to be measured, this process is often divided in two or more steps. Usually the measurements produced in the first step help select the optimal set of patches to be measured in the main step, while the measurements in the last step may help verify the quality of color reproduction. The measured data can then be used to produce the color profile (a family of lookup tables used to control the amount of ink necessary for the best possible color reproduction).
In the prior art, the entire procedure of getting a color profile, including printing the patches, transferring them to the measuring table, taking the measurements with a specialized colorimeter, and then processing measured data involved a number of manual steps and was known to be labor intensive and error prone. Therefore, in order to improve the actual accuracy of color printers it was highly desirable, if not required, to automate this procedure, expanding the color printing software to include the color measurements in the automated loop.
In the recent prior art, some of the leading-edge printers, in particular those used for wide format printing, have already been equipped with optical sensors such as CCD arrays used in digital cameras, in particular in machine vision applications. These optical sensors have been previously used to determine positional accuracy of graphical elements such as dots and lines, and were not intended to be used for color measurement purposes.
Thus, a need exists in the prior art to automate several previously unrelated steps toward creating specialized, custom color transforms which are ink and media dependent (and which also account for a majority of other factors that effect print quality, such as humidity/temperature conditions). Thus, a need exists in the art to provide customized, optimum printing of inks onto a large variety of printing substrates using optimized color transforms without resorting to additional equipment or manual steps to create such transforms. Further, a need exists in the art of digital ink jet printing to compensate for color image fidelity by revealing the actual chromatic characteristics of patches made up of individual colored dots and then creating custom color transforms that inherently include the actual color xe2x80x98performancexe2x80x99 of the inks on the media (which is often not a xe2x80x98truexe2x80x99 reference white) colored ink droplets in order to improve the quality and the visual clarity of text, graphics, and color appearing on the print media. Finally, a need exists in the art to improve the quality of output by automatically synchronizing ink and media printing components so that a plurality of colored ink droplets accurately record dots of desired color upon desired locations on the printing media to thus rapidly form high quality printed output closely resembling original source images.
The method and apparatus of the present invention increases the precision for controlling a plurality of cartridges that emit an extremely wide variety of colored ink droplets from several ink jet print cartridges in a digital print engine. The present invention addresses performance confirmation of individual nozzles and registration among each cartridge, and hence, each nozzle to each other nozzle, by utilizing a charge coupled device (CCD) and an array of carefully chosen light emitting diodes (LEDs) to create an optimal CCD sensor signal to quickly and accurately locate patterns of individual dots created by the droplets emitted from each of thousands of nozzles disposed in said print cartridges. The LED array provides a source of illumination upon a portion of a printed substrate within the field of view of a compact imaging subassembly which includes a CCD, a lens, and the plurality of LEDs which supply illuminating radiation from 400 to 700 nanometers wavelength (the xe2x80x9cvisible spectrumxe2x80x9d).
Thus, since the individual dots are of a wide variety of colors, LED operation preferably includes a control circuit that references a memory storage device which indicates the color of the ink in each cartridge. In the preferred embodiment, twelve (12) cartridges each having three hundred (300) ink emitting nozzles are disposed in a reciprocating carriage that traverses a printing zone of approximately four to six feet (4xe2x80x2-6xe2x80x2) in length. Even assuming that not every nozzle is used during printing on the order of three thousand individual nozzles can emit as many as twelve different colors of ink. Manual registration or validation of nozzle performance, while possible, is clearly not a time or cost conscious alternative when handling such a large variety of inks expelled from so many individual cartridges.
A series of electronic images are recorded during said periodic illumination and each may be stored, compared to a corresponding series of reference dot patterns, then used for updating an electronic printing sequence, and/or viewed on a monitor to confirm orientation and location of the optical sensor with respect to individual dots. The electronic image is typically temporarily stored as a two dimensional bit map in a portion of a memory storage device that may include location, size, and color information of each individual dot interrogated and successfully detected by the optical sensor. The source of illumination may comprise many different colored source elements, such as red, green, blue (RGB) which taken in the aggregate preferably cover the entire visible spectrum.
The present invention thus finds increased utility over a variety of prior art printing methods and platforms to achieve accurate placement and registration among a plurality of different color ink droplets recorded on a variety of desired pre-selected locations of a printing media. By sensing dot patterns produced by one or more print head cartridges with a first print nozzle control sequence and then determining which of a variety of controlled parameters to adjust to improve registration first among nozzles of each cartridge with respect to each other and thereafter among nozzles of different cartridges. In a preferred embodiment of the present invention, a print engine employs several print heads that can readily provide nozzle redundancy so that mis-firing and non-firing nozzles may be compensated and replaced by fully operational nozzles without degradation of image or needless loss of available printing time. The initial steps of the inventive method herein preferably include conducting compensation calculations based upon the location of discrete colored dots recorded on the print media, which calculations are promptly implemented in an amended excitation control sequence prior to initiating later calibration steps so that successively finer tuning for dot placement accuracy results.
The present apparatus includes an optical sensor for sensing and storing information about dots recorded on a print media by said print heads wherein the optical sensor is preferably coupled to the carriage assembly, and based upon each of several iterative steps where differing calibration patterns are optically sensed, achieves highly accurate registration among the print heads.
One way to determine the color of a colored patch is to measure its reflectivity at a sufficient number of narrow wavebands spanning across the visible spectrum. This is usually done by comparing the amount of light reflected by a color patch with that reflected by a white patch. The comparison can be done using the values of electric signals generated by a photosensor such as a photodiode or CCD (which is an array of miniature photodiodes). One difficulty here is that the electric output of most inexpensive photosensors is not sufficiently linear (with respect to the energy of incoming light) to provide data with accuracy suitable for color measurements. Another difficulty lies in the fact that in reality, color swatches produced by the printer are just an assembly of separate or overlapping dots made by drops of just a few different inks. To extract reflectivity from CCD measurements, each CCD pixel output received with certain exposure time xe2x80x98txe2x80x99 can be first mapped to the exposure time xe2x80x98Txe2x80x99 that was required to get the same output in the white patch. The reflectivity xe2x80x98Rxe2x80x99 then can be determined as a ratio of T/t averaged over CCD pixels, that is: R= less than T greater than /t.
An exemplary carriage assembly, print head sockets and related mounting and electrical coupling of a preferred embodiment are fully described in U.S. Pat. No. 6,290,332 entitled xe2x80x9cCarriage Assembly For A Large Format Ink Jet Print Engine,xe2x80x9d by Crystal, Gonier, and Barclay, the contents of which are incorporated herein by reference in their entirety.
The following figures are not drawn to scale and only detail a few representative embodiments of the present invention, more embodiments and equivalents of the representative embodiments depicted herein are easily ascertainable by persons of skill in the digital imaging arts, and are expressly covered hereby. The inventors reserve the right to augment or otherwise render any portion of the written description, and those aspects inherent therein and known to those of skill in the art, as illustration(s) hereof.